The present invention relates to a low friction coating and to the deposition of low friction coatings by vacuum ion and plasma techniques. The coatings have non-stick properties, low hydrophilia and high stability.
The disulphides of elements such as Mo and W are known to have very low friction properties due to their unique chemical bonding and structure. MoS2 and WS2 coatings are currently being deposited by vacuum ion and plasma techniques such as magnetron sputtering (MS), plasma assisted chemical vapour deposition (PACVD) and ion beam assisted deposition (IBAD). MoS2 and WS2 coatings have been used in tribological applications as a solid lubricant in aerospace products [M. R. HILTON, P. D. FLEISCHAUER, Surface and Coating Technology, 68/69 (1994) 398; J. S. PRZYBYSZEWSKI, T. SPALVINS, Nasa T N D-5349, July 1969] and other engineering fields such as cutting applications [J. RECHBERGER, R. DUBACH, Surface and Coating Technology, 60 (1993) 393].
The deposition process has always been subject of poor reproducibility. Different techniques have been applied in order to enhance the reproducibility of coating properties, among them the inclusion of other elements in the structure [M. R. HILTON, Surface and Coating technology, 68/69 (1994) 407; B. S. STUPP, thin Solid Films, 84 (1981) 257]. In certain cases the induction of other elements enhanced the coating properties.
General problems inherent to these family of coatings are their thermal and atmospheric instability. The coatings react with water vapour and oxygen transforming the sulphide into an oxide with very different tribological properties. In addition to these problems the maximum useful thickness for the coating has been always under 2 xcexcm. Thicker coatings tend to suffer severe cracking under working pressure conditions.
Tungsten disulphide films have had their tribological properties improved by incorporating CFx into a mixture of tungsten and sulphur by pulse laser deposition [Surface and Coatings Technology Col 76-77, 1995 400-406].
Similarly, fluorinated graphite containing 10-40% F has been added in mixtures to MoS2 to improve wear resistance [SU 601306].
However there remains a need for coatings with improved properties which overcome the limitations of present compositions.
According to the present invention there is provided a metal sulphide coating composition characterised in that the composition further comprises silicon and fluorine.
According to a further aspect of the present invention there is provided a method of depositing a low friction metal sulphide coating onto a substrate by a vacuum ion or plasma technique characterised in that silicon and fluorine or precursors thereof are introduced into the deposition unit.
According to yet a further aspect of the present invention there is provided a product coated with a metal sulphide coating composition of the present invention or a product produced by a method of the invention.
Preferably the deposition of coatings is by vacuum ion or plasma techniques such as MS, PACVD, IBAD, electron cyclotron resonance (ECR), arc evaporation (AE), electron beam evaporation (EBE), laser ablation (LA), ion implantation (II), or combinations of these techniques.
More particularly the coatings comprises
(a) one or more of the following elements: Mo, W, Nb, Ta, Ti, Zr, Hf
(b) sulphur
(c) fluorine,
(d) silicon
and optionally
(e) one or more of the following elements: C, B, Al, V, Cr, Fe, Co, Ni, Sm, Au, Cu, Zn, Sn, Pb, N, H, O
In one embodiment the invention relates to the deposition of a film in which at least one volume, no matter its size, comprises a chemical composition, as either a single or a plurality of phases, of the following formula:
MxRySzSivFw 
where
M represents one or several elements as stated in (a)
S represents the sulphur element;
Si represents the silicon element;
F represents the fluorine element;
and R represents one or more of the elements described in (e).
The values of x, y, z, v and w are within the ranges (by atomic ratio) of:
x=0.2 to 1.5
y=0.01 to 4, or y=0 to 4
z=0.2 to 6
v=0.02 to 3
w=0.01 to 6.
Examples of chemical compositions for coatings described by this invention include:
In addition the process could be carried out in different vacuum conditions of gases and pressure. Noble gases (He, Ar, Kr, Xe, Rn) or reactive gases (e.g., H2, O2, N2, SF6, Si2F6) or a mixture of them could be used during the deposition process. The process could also be carried out in ultra-high vacuum, without the assistance of any or very limited gas or vapour sources. The metal elements cited in (a) could have been introduced by different means such as thermal evaporation, arc evaporation, electron beam evaporation, laser ablation, magnetron sputtering, plasma assisted chemical vapour deposition, ion beam assisted deposition, ion implantation, which could use different sources for the elements such as pure metal target (e.g., Mo, W, Nb, Ta, Zr, Hf, Si), alloys (e.g., Mo/W, Mo/Ti/Zr, Mo/Ti, Zr/Ti) and compounds either solid (e.g. MoS2, WS2, Mo2C, WSi2, WCxe2x80x94Co, WCxe2x80x94Ni), liquid (e.g. WF6, MoF6) or gas (e.g. W(CO)6) or any combination of them.
The sulphur could be introduced by the same or different means as stated for the element of section (a), which could use different sources such as pure sulphur (e.g. S8), metal sulphide (e.g. MoS2, WS2, MoWS) or other sulphur compounds (e.g. SF6) or any combination of them.
The fluorine could be introduced by different means using different precursors such as F2, SF6, C2F4, CF4, C2F6, WF6, MoF6, Si2F6, BF3, NF3, or any combination of them.
The silicon could be introduced as a pure element or as a compound, for example, Si2F6, Si3N4, SiC or any combination of them.
Elements cited in (e) could be introduced by similar means as (a), (b),(c) and from appropriated sources (e.g. C from C targets or from C2H2, C2F4 gas, N from N2 or NF3) or any combination of them.
All the elements present within the coatings described in this invention could be incorporated within the coatings in an homogeneous or inhomogeneous way. The coating described in this invention could be a part or the whole of the total deposited film. The composition of the coating could be homogeneous all the way from the surface to the interface coating-substrate. The coating could be deposited on to other layers deposited by same, similar or different means and same, similar or different chemical composition and/or structure. The coating could vary its composition from the surface towards the interface. The coating could be a partial or total periodic repetition of different layers. The coating could be made of different layers without periodic repetitions. The coating could also be subjected to further vacuum and non-vacuum treatments which could imply changes in its original deposited chemical composition and/or structure (e.g. thermal treatment, chemical or electrochemical treatments, radiation or ablation treatments). These treatments could be also included as a part of the general deposition process.
The coating could be produced on one or several in-line deposition units or on one or several isolated deposition units.
The deposition unit could comprise one or a plurality of coating means and/or sources. Samples to be coated could be static or dynamically moved in the deposition unit.
The coatings described by the present invention have a low friction coefficient.
The coatings described by the present invention have lower hydrophilia than the standard metal disulphide coatings produced previously.
The coatings described by the present invention have good thermal and atmospheric stability which are improved compared to other disulphide coatings.
The coatings described by the present invention have non-sticky properties.
The coating described by the present invention have good tribological properties.
The coatings described by the present invention do not suffer severe cracking during working pressure conditions improving the actual limits of the actual disulphide coatings.
All these properties make possible the use of the coating described by this invention in the following applications:
Optic and magnetic recorder media.
Aircraft and spacecraft bearings.
Ball bearings, ball screw, gears, cam shafts, valves, fuel injectors, oil and combustion pumps, cylinders and piston rings, as an example in the automobile and other motor industries.
Cutting and forming tools such as drills, end mills, inserts, saws and other tools used in the machining of aluminium, aluminium alloys, copper and copper alloys, inserts, precious metals (e.g. gold, silver, osmium, iridium, platinum, ruthenium, rhodium and palladium), steel, stainless steel, carbon fibres, glass fibres, ceramics, metal matrix composites, organic matrix composites, wood, cardboard, plastics and polymers (e.g. plastic packing) or combinations of such materials such as cardboard plus polymer (e.g. tetrabrick packing), aluminium plus polymer (e.g. drink cans), steel plus polymer (e.g. food tins).
Stamping, punching and conforming operations in materials as described in the previous group.
Coating of mould components such as the mould, injectors, nozzles and valves as an example to enhance demoulding and wear protection.
Operations in textiles and paper industry related to guiding, sliding, machining, cutting, stamping, printing, improving the quality and wear resistance of the tools and elements.
The invention will be further described by way of example only with reference to the following figures in which: